On-Demand Ammonia Capture and Separation with Open Aluminophosphate Frameworks

The development of adsorbents for ammonia capture and separation is a challenging task, with significant industrial relevance. We report herein, for the first time, open aluminophosphate frameworks for on-demand ammonia adsorption. Under ambient conditions, the AlPO-18 molecular sieve exhibits exceptional dynamic ammonia uptake of 3.36 mmol/g at an extremely low ammonia concentration of 200 ppm, showing great potential as a disposable adsorbent for trace ammonia capture. AlPO-11 molecular sieve manifests a desirable ammonia dynamic uptake of 5.73 mmol/g at high ammonia concentration of 50,000 ppm, with excellent recyclability upon regeneration under mild conditions. Aspen simulations demonstrate the advantage of the adsorptive separation process employing AlPO-11 adsorbent for ammonia recovery, with huge energy savings of 51% in comparison with the compression-based process currently employed in the ammonia synthesis industry. The underlying mechanism of ammonia adsorption and the nature of ammonia binding in aluminophosphate frameworks are fully elucidated by employing a combination of X-ray photoelectron spectroscopy, in situ Fourier transform infrared spectroscopy, quasi in situ magic angle spinning nuclear magnetic resonance, and density functional theory calculations. The coordination of ammonia on the framework aluminum sites in aluminophosphates is clearly disclosed. The development of low-cost and commercially available aluminophosphate molecular sieves may unlock their potential for on-demand ammonia capture and separation in various scenarios.